Optical disks are widely used as external storage media for computers. Magneto-optical disks have become popular because they are rewritable and provide a relatively high recording density. In the past, 3.5 inch magneto-optical disks were only capable of storing 128 MB of information. Recent advances, however, have enhanced the storage capacity of 3.5 inch magneto-optical disks to 1.3 GB, and even greater increases in storage capacity are presently being sought.
Magneto-optical disks include at least one recording layer formed on a substrate. Information is recorded/reproduced from magneto-optical disks using a laser light source and a magnetic source. Typically, grooves (tracking guide grooves) are formed in spiral fashion on the substrate of the medium. Data is recorded on tracks provided on lands between these grooves.
In the past, the recording density of magneto-optical disks has been limited by the diameter of the beam spot of the laser beam. However, in recent years, magneto-optical super resolution technology known as MSR (Magnetically Induced Super Resolution) has facilitated the recording and retrieving of a mark smaller than the diameter of a laser beam. For example, with a 3.5 inch magneto-optical disk, recording and retrieving of a mark smaller than the laser beam spot with track pitch of 0.90 μm and a mark length of 0.38 μm is now possible. Consequently, a ten fold increase in storage capacity to 1.3 GB has been realized.
Magneto-optical disks record and retrieve in a storage unit termed a sector. By manner of illustration, FIGS. 12A and 12B show a traditional MSR magneto-optical disk sector format in which sector address information portion 90 is physically formed as an indented (embossed) pit with a stamper in the same manner as a tracking groove. Sector address information 90 includes a sector mark SM which indicates the beginning of a sector, PLL phase lead-in term signal VFO1, address mark AM indicating the beginning of the first sector ID, first sector address ID1, PLL phase lead-in term signal VFO2, address mark AM indicating the beginning of the second sector, second sector address ID2 and post amble PA indicating the end of the sector address information portion.
Sector address information stored in ID1 and ID2 includes track number and sector number information. The second sector address ID2 stores the same information as the first sector address, and is included as a backup in case ID1 becomes unreadable.
A gap 91 separates the sector address information portion 90 from VFO area 92 in which a VFO pattern for adjusting the frequency is recorded. Sync byte area 93 is interposed between a data area 94, and the aforementioned VFO area 92. A post amble (PA) 95 and buffer 96 for a buffering area are formed subsequent the data area 94.
Data is recorded at a high density in the data area 94 portion using MSR techniques. In contrast, sector address information portion 90 is recorded at a significantly lower density than the data recorded in the data portion 94, since it is physically formed by embossing or the like.
Accordingly, one problem associated with conventional magneto-optical storage mediums relates to the relatively large area required to store sector address information, and the ensuant decrease in usable storage capacity of the medium.